FIG. 1 shows an organic thin-film transistor (OTFT) 1 of prior art, having a so-called “high-gate” and “low-contact” structure. Transistor 1 comprises a lower substrate 2, having two electrodes 3, 4, respectively a source electrode 3 and a drain electrode 4, formed thereon. A semiconductor layer 5 is deposited on lower substrate 2, and on source and drain electrodes 3 and 4. Finally, a dielectric layer 6 is deposited on semiconductor layer 5, and has a gate electrode 7 formed thereon.
The transistor effect is obtained, as known per se, by applying a voltage between gate electrode 7 and lower substrate 2, to create, in semiconductor layer 5, a conduction channel between source electrode 3 and drain electrode 4.
However, the performance of such organic transistors strongly depends on the chemical characteristics of the interfaces between semiconductor layer 5, dielectric layer 6, and source and drain electrodes 3 and 4.
Indeed, the electric permittivity of the flexible substrates, forming lower substrate 2, such as polyethylene naphthalate (PEN), is generally greater than 3 (approximately 3.5), thus generating an electric stress in semiconductor layer 5 and/or creating a trapping of charges at the lower substrate/semiconductor layer interface (also called leakage path), which strongly disturbs the performance of such “high gate” organic transistors.
Flexible polyethylene naphthalate (PEN) substrates have at their surface a high concentration of dipoles (acid COOH group, polar OH− group, fluorinated group) which are electrically non-neutral (positive or negative charges) and which strongly alter the electric conduction in semiconductor layer 5 of such organic transistors.
To overcome the problems of interface and of high permittivity of lower substrate 2, it is known to deposit a self-assembled monolayer SAM between lower substrate 2 and semiconductor layer 5 and source and drain electrodes 3 and 4, and this, to limit the influence of substrate 2.
Such an organic field-effect transistor structure is particularly described in publication “Influence of Substrate Surface Chemistry on the Performance of Top-Gate Organic Thin-Film Transistors”; Boudinet D, Benwadih M, Altazin S, Verilhac J M, De Vito E, Serbutoviez C, Horowitz G, Facchetti A; J Am CHEM SOC, 2011 Jul. 6; 133 “26”; 9968-9971, EPUB 2011 Jun. 10, PMID; 21661723 [pubmed].
FIG. 2, which also describes a prior art field-effect transistor, shows the structure of the transistor of FIG. 1, further comprising a self-assembled monolayer 8 positioned between lower substrate 2 and semiconductor layer 5 and source and drain electrodes 3 and 4.
Several trials have been carried out with a semiconductor layer 5, referring to FIG. 4, made of a derivative of N,N′-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (n-channel), sold under trade name Activink N1400 by POLYERA, or of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] called poly(triaryl amine) or PTAA (P-channel) and several self-assembled monolayers (SAM), referring to FIG. 3, comprising alkyl/phenyl-amino (examples 1 et 2), alkyl (examples 3 et 4) and halo-alkyl (examples 5 et 6) functional groups.
As can be seen in FIGS. 5A and 5B, the electric performance of such organic transistors is altered or shifts whatever the nature of semiconductor layer 5 and/or the nature of self-assembled monolayer (SAM) 8.